Residue
Singularity:
A singularity is a point at which a function becomes undefined, infinite, or behaves in a way that prevents the function from being smooth or continuous.
Isolated Singularity:
An isolated singularity is a singularity that is distinct and separated from other singularities. In other words, it is a point where the function misbehaves, but if you look in a small neighborhood around that point, the function is well-defined and analytic (smooth).
Types of Isolated Singularities:
Removable Singularity: The function can be defined at the singular point in such a way that it becomes analytic in the entire neighborhood. The singularity is "removed" by defining the function appropriately.
Pole: The function approaches infinity as you get closer to the singular point.
Essential Singularity: The function has a more complicated behavior at the singular point, and it cannot be "smoothed out" or made analytic in the neighborhood.
Examples:
For the function
�
(
�
)
=
1
�
f(z)=
z
1
, the point
�
=
0
z=0 is an isolated singularity, and it is a simple pole.
For the function
�
(
�
)
=
�
1
�
g(z)=e
z
1
, the point
�
=
0
z=0 is an isolated singularity, and it is an essential singularity.
Behavior in a Neighborhood:
The term "isolated" emphasizes that the singularity is not part of a cluster of singularities; it stands alone. The function behaves well in a small region surrounding the isolated singularity.
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